The present invention relates to cooling within integrated circuit (IC) packaging structures. More particularly, the present invention is directed to cooling of integrated circuit chips using a thermally conductive conformable material.
As heat is generated during the functioning of integrated circuit chips (ICs), the thermal resistance to the heat sink must be as small as possible so that the operating temperature of the chip is low enough to assure the continued reliable operation of the device. The problem of heat removal becomes ever more difficult as chip geometries are scaled down and operating speeds are increased, resulting in increased power density. The ability to adequately cool the chips is therefore a limiting factor in the further increase of system performance. Multiple terms are used in the art to describe the elements of the package which are used to remove heat, including heat sink, heat spreader, cooling plate, and heat transfer surface. In an array of ICs mounted on a substrate such as a mutichip module (MCM), special cooling difficulties are presented. In an MCM, the chips may be mounted very close together and nearly cover the whole top surface of the MCM. With such an arrangement, it may not be possible to use a heat spreader bonded directly to the back surface of the chips, as is sometimes used for isolated chips to reduce the heat flux (power/unit area, i.e. W/cm2).
A common technique for removing heat from high-power ICs makes use of a cooling plate or heat sink which is thermally attached to the chip using a thermally conductive material such as a thermal paste or thermal grease. Heat is removed from the cooling plate or heat sink by methods such as forced air cooling or circulating liquid coolants. The term cooling plate will be used generically herein to refer to either a heat sink or a cooling plate. In heat removal techniques, it is critical to minimize the thermal resistance between the chip and the cooling plate or heat sink. The present invention is directed to reducing this thermal resistance.
Various approaches are set forth in the art to achieve cooling of ICs mounted on substrates. For example in U.S. Pat. Nos. 5,239,200 issued Aug. 24, 1993 to Messina et al., in U.S. Pat. No. 5,177,667 issued Jan. 5, 1993 to Graham and Moran, and in U.S. Pat. No. 4,500,945 issued Feb. 19, 1985 to Lipschutz, each of which is assigned to the present assignee, the use of circulating liquid or gas coolant is described. U.S. Pat. No. 5,023,695 issued Jun. 11, 1991 to Umezawa et al., describes the use of a circulating cooling fluid in conjunction with a cooling plate having cut cavities. Circulating fluid coolants or forced air cooling are required to remove heat from the surface of the external module, which is also referred to as a heat sink or cooling plate. The present invention is directed to reducing the thermal resistance between the chip within the package, whether the package is an MCM or an SCM, and the external module surface.
A thermally conductive paste or similar conformable compliant thermally conductive material is typically placed between the IC chip and the cooling plate or heat sink. Thermally conductive paste typically comprises thermally conductive particles having a distribution of sizes dispersed within a binder material or matrix, such as the paste described in U.S. Pat. No. 5,098,609 issued Mar. 24, 1992 to Iruvanti et al. In the ""609 patent, paste is applied between the top of the IC mounted on the substrate and the lower flat surface of a cooling plate facing the substrate. The type of paste described in the ""609 patent can be used in the present invention, as can other thermally conductive pastes used in the art or other compliant particle-based conformable materials. Typical particle-based materials include those having a wax matrix, commonly known as phase-change materials, those having a silicone-based matrix, and dry particle lubricants such as graphite and metal powders.
When applying a thermal paste between the back of a chip which is electrically attached to a substrate and the lower surface of a cooling plate, the paste layer must be made as thin as possible in order to reduce the thermal resistance through the paste layer. The paste must also be compliant, or flexible, maintain its integrity, surface adhesion and chip coverage despite the expansion and contraction of the packaging structure caused by power and temperature cycling. U.S. Pat. No. 6,091,603 issued Jul. 18, 2000 to Daves and Edwards and assigned to the present assignee, describes the use of a customized deformable lid understructure which permits a reduction in the amount of thermally conductive material in the primary heat dissipation path. FIG. 1, taken from the prior art ""603 patent, shows a multichip module in which chip 600, mounted on chip-carrying substrate 500 by solder bumps 650, is thermally connected to deformable lid understructure 103, with which it lies in parallel using thermally conformable material 200. Deformable lid understructure 103 and chip 600 lack the microstructure of the present invention, employing instead deformable lid understructure 103 to reduce the thermally conductive paste thickness, resulting in improved heat dissipation.
Whenever a particle-filled paste is used between a flat cooling plate and a flat chip substrate, the thickness of the thermal paste layer is limited by the size of the largest particle present in the paste. Applying pressure to try to reduce the thickness of the paste layer risks cracking of the chip due to the concentration of pressure falling on the largest particles in the paste. However, it is desirable to have a range of particle size in the paste in order to improve the solid packing density. Separating the largest particles from the paste by sieving is theoretically possible, but impractical because as the particle size is further reduced, it becomes increasingly difficult and expensive to sieve out the particles which exceed the desired size range.
An additional difficulty observed with the use of thermal paste is the migration of the paste from behind the chip and the formation of voids due to differential thermal expansion of the various parts of the package during thermal cycling. Such paste migration can greatly increase the thermal resistance between the chip and the cooling plate during the lifetime of the electronic package, possibly causing catastrophic heating and destruction of the chip.
Several approaches in the art describe providing an altered surface of the cooling plate which is in contact with thermal paste. In U.S. Pat. No. 5,825,087 issued Oct. 20, 1998 to Invanti et al. and assigned to the present assignee, the cooling plate used in conjunction with a thermal paste or a thermal adhesive has been roughened by grit blasting or provided with a plurality of crisscrossing channels in order to improve the adhesion of the thermal medium and inhibit its flow during operation of the electronic module. FIG. 2, taken from the prior art ""087 patent, shows in detail the channels 18 and corresponding protrusions 17 of roughened. area 16 of cooling plate 14 in which heat generated on chip 13 is removed by transfer through paste 15. The purpose of roughening the surface of plate 14 is to inhibit the movement of paste 15. U.S. Pat. No. 5,345,107, issued Sep. 6, 1994 to Daikoku et al. describes the use of a grooved solid body in conjunction with thermally conductive fluid or thermally conductive grease and a low-pressure spring to hold the solid body in close contact with the electronic device. In FIG. 3, taken from the prior art ""107 patent, the excess capacity of grooves 40 and 41 on heat transfer surface 100 of solid thermal conductor 33 enables closer contact of the thermal conductive grease 11 (not shown in this figure) between heat transfer surface 100 and the heat transfer surface of the chip carrier 101 (not shown in this figure) when the pressure of a spring 34 (not shown in this figure) is applied. The presence of grooves is intended to prevent the thermally conductive medium from migrating to surfaces outside the device during power cycling and to allow the thickness of the thermally conductive medium to be reduced. In the structure described in the ""107 patent, the minimum paste thickness is still limited by the largest particle size. In the present invention the average thickness of the paste applied between the nominally parallel cooling plate and chip is reduced below the size of the largest particle in the paste by creating discrete depressions into which the largest diameter paste particles are redistributed under pressure, facilitating cooling. At the same time, the presence of part of the largest particles above the discrete depressions inhibits lateral movement of smaller particles in the paste layer.
In FIG. 4, taken from prior art U.S. Pat. No. 5,298,791 issued Mar. 29, 1994 to Liberty and Jones, is described the use of a thermally conductive perforated, grooved or embossed sheet which is provided with pressure-sensitive adhesive film 11 on the major surfaces in order to bond one major surface to a heat source in a chip and to bond the other major surface to a heat sink (not shown in this figure) with which it is in contact. The function of the perforations, grooves or embossing is to remove air between the film and the flat surfaces with which it is in contact upon the application of pressure during the bonding process, a problem not seen in the present invention.
None of the prior art addresses the same problem in the same way to obtain the same result as in the present invention.
For the packaging of high performance integrated chips, it is necessary to further reduce the thermal resistance along the primary heat dissipation path from the chip, where the heat is generated, to the external surface of the module, where the heat is removed. This goal must be accomplished in a manner consistent with ongoing packaging requirements such as providing high speed and dense electrical interconnects, protection from the environment, low cost and high reliability. In high end servers which use multichip modules (MCMs), the cooling of chips mounted on the module is a key system constraint. About 70% of the thermal drop between the chip surface and the MCM heat sink is across thermally conductive paste when heat spreaders are not used. The thermal paste mediates the thermal expansion mismatch between the chip, which typically comprises silicon, and the thermal hat or cap, which is typically made of copper or aluminum. If a heat spreader is used, the fraction of the thermal drop across the paste layer is reduced, but many system designs do not have enough room on the MCM to permit the use of heat spreaders, and heat spreaders are an added cost. For single chip modules (SCMs) which mount a single high power chip, a heat spreader, also known as a thermal spreader, having a thermal coefficient of expansion close to that of the chip is attached to the back of the chip using a material such as a silver filled epoxy or solder which is noncompliant and nonconformable after curing or solidification. A layer of thermal paste, or other thermally conductive conformable material, is then used between the thermal spreader and the thermal hat, also known as a thermal cap or heat sink. With each succeeding CMOS generation the power density (W/cm2) of the processor chips has increased, and that trend is likely to continue apace as chip circuit density and clock frequencies are further increased. The traditional means of reducing the thickness of the thermal paste or attaching heat spreaders directly to the processor chips are not adequate for projected future servers. The ability to reduce the thickness of the thermal paste is limited by the size of the largest of the particles present in the paste. Attempting to reduce the thickness of the paste to a thickness below that of the largest particles by increasing the applied force results in a concentration of stress which can crack the chip.
In the present invention a discretely shaped microstructure is provided on at least one of the surfaces in contact with a compliant, or flexible, thermally conductive conformable material, such as a thermal paste. When pressure is applied, the largest particles within the paste, facilitated by the slope of the walls within the microstructure, are preferentially directed toward the floors in the microstructure.
The result of the migration of the largest particles into the microstructure is that the average thickness of the paste is reduced, and hence the thermal resistance of the paste layer and temperature drop across the paste layer are reduced. The thermal conductivity across the paste thickness can be represented as a parallel resistor network. Therefore a microstructure in a corrugated or rectangular pattern in which the average thickness of the paste is the same as that of a planar structure would have a lower thermal resistance, especially if the paste is very thin directly above the projecting flat surfaces between cells. The thickness of the paste could be further reduced in combination with reducing the size of the largest particles within the thermal paste as manufactured.
The present invention does not require the fabrication of additional parts within the IC chip or on the module. Process and structure changes pursuant to creating the discrete microstructure are minimal. In an embodiment of the invention the microstructure pattern is readily formed on the back surface of a silicon chip by anisotropic wet etching. In a further embodiment in which a thermal spreader is used, the microstructure pattern is formed on the surface of the thermal spreader in contact with the thermal paste layer by using a master as a mold during the formation of the thermal spreader. Alternatively, the microstructure can be formed on the heat sink side of the thermal paste layer by a process such as electroforming from an etched silicon, or other, master.